Recently, vehicles propelled by at least the drive power of a motor, such as electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), fuel cell vehicles (FCV), or the like, have a battery assembly housed in a battery box, which serves as an electric power source for an electric motor. The battery assembly is made up of a plurality of series-connected battery modules, each of which comprises at least one cell as a secondary cell that is chargeable and dischargeable. The electric motor is energized by a high DC voltage generated by the battery assembly and which is applied through an inverter.
The series-connected battery modules initially are charged to equal levels. However, when the battery modules are repeatedly charged and discharged, the charge levels of the respective battery modules gradually become different from each other as a result of characteristic variations or the like of the battery modules.
Usually, a charging process is performed on the battery assembly. When any one of the battery modules of the battery assembly has reached an upper charge-level limit, the charging process on the battery assembly must be stopped, even if the other battery modules have not yet been fully charged.
On the other hand, when any one of the battery modules of the battery assembly has reached a lower charge-level limit (end-of-discharge voltage) during a discharging process performed on the battery assembly, the discharging process also must be stopped.
As a vehicle carrying such a battery assembly travels over a greater distance, or in other words as the vehicle is used for a longer period of time, the overall series-connected array of battery modules, i.e., the battery assembly, tends to reach the upper charge-level limit or the lower charge-level limit earlier than before, so that the charging capacity of the battery assembly becomes reduced as a practical matter.
In view of the aforementioned drawbacks, techniques have been proposed for uniformizing the remaining capacities of the battery modules that make up a battery assembly, by monitoring voltages that represent the remaining capacities of the battery modules, discharging battery modules that have relatively high voltages, and charging battery modules that have relatively low voltages with the discharged electric power. See, Japanese Laid-Open Patent Publication No. 2010-213474 (JP2010-213474A) (Abstract, FIGS. 1, 2, and 3) and Japanese Laid-Open Patent Publication No. 2011-067021 (JP2011-067021A) (Abstract, FIGS. 2, 9, and 10).
According to the techniques disclosed in JP2010-213474A and JP2011-067021A, AC generating circuit blocks and DC generating circuit blocks are connected to the battery modules of the battery assembly, and the circuit blocks are interconnected by capacitors (JP2010-213474A) or by series-connected circuits made up of capacitors and inductors (JP2011-067021A) for adjusting the charge levels of the respective battery modules.
For facilitating maintenance and servicing of battery assemblies, it has been recommended to provide a manual cutoff switch at a central point, for thereby halving the voltage across a battery assembly made up of series-connected battery modules. See, GUIDELINES FOR ELECTRIC VEHICLE SAFETY—SAE J 2344 JUN 1998 (4.3.2.1 Suggested Disconnect Location and Type).